Bernd Helmle <mailings at oopsware.de> writes:
> The Slony-I documentation states the following:
>
>
> "This points to it being a bad idea to have the large communications
> network resulting from the number of nodes being large. Up to a half
> dozen nodes seems pretty reasonable; every time the number of nodes
> doubles, this can be expected to quadruple communications overheads."
>
> Consider a setup with dozens of very small nodes which aren't heavliy
> frequented. Neither of these nodes is considered to be a forwarder and
> stores only information which aren't updated frequently (so we
> consider a small database between 20 - 30 MB).  The Lagtime can be
> minutes (we don't care wether 5 or 15 minutes). The network is stable.
>
> I wonder wether the number of nodes can be significantly larger than
> the half dozens the documentation mentions, especially if you don't
> need to use Failover, MOVE SET and you are able to use high SYNC
> timeouts values. For example, this can be configuration clusters of a
> large network where configuration changes needs to be propagated
> through a large number of nodes.
>
> A customer is planning such a setup and they consider Slony-I exactly
> for this purpose since they are impressed about its
> reliability. However, because communications cost is quadratic, there
> can be significant network traffic.
>
> Opinions? Experiences?

I suppose that if you can afford the cost, then all is well.

We have been cutting down on the costs, over time, not to the
fundamental degree of it no longer being quadratic in expansion, but
rather, but diminishing quantities of extra work.

For instance, in v2.0, sequence updates only take place when their
values actually change, which should greatly reduce the cost of having
many sequences replicated, particularly if only a few are heavily
used.

If you have a system that is relatively lightly loaded, then it may be
OK if replication is *relatively* pretty expensive, as long as it's
not actually too expensive for your resources to cope with.
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